Activated photoconductors and recording element therewith



United States Patent 3,352,671 ACTEVATED PHOTOC'QNDUCTORS AND RECORDHNGELEMENT THEREWITH Michael Michalchik, Bethpage, N.Y., assignor toFairchild Camera and instrument Corporation, Syosset, N.Y., a

corporation of New York No Drawing. Filed Dec. 28, 1964, Ser. No.421,631

17 Claims. (Cl. 961.8)

This invention relates to electrophotography. More particularly, itrelates to electrophotographic compositions used for coating a basematerial such as paper or metal, and the recording elements thusproduced. It rclates also to amine activated photoconductors.

The art of electrophotography depends principally on the photoconductiveproperties of metals and metallic compounds. These include, for example,the oxides, sulfides, iodides, selenides and tellurides of zinc,mercury, aluminum, antimony, bismuth, cadmium and molybdenum; metalliclead and selenium; arsenic trisulfide, lead chromate and cadiumarsenide. By far the most widely employed photoconductive substance iszinc oxide.

Electrophotography has been applied to many purposes. Dry reproductionof originals in ofiice copying by the well known electrofax process isone example. Lithography, photoengraving and production of electricalprinted circuits are others.

For convenience this invention will be principally described as appliedto the production of lithographic plates. Those skilled in the art willrecognize that it is also applicable to other electrophotographicprocesses such as those mentioned above. Accordingly, the invention isnot limited to the production of lithographic plates.

In the preparation of a lithographic plate, a recording element is firstproduced by coating 21 plate of zinc, magnesium or other etchable metalwith a coating com position comprising an electrically insulating filmin which a photoconductive substance such as zinc oxide is suspended.Photoconductive zinc oxide is well known. It generally has a surfacephotoconductivity of at least about ohms /square/watt/cm. when exposedto a wave length of 3900 A.

" The coating is then made sensitive to light by substantially coveringthe surface with an electrostatic charge in the dark. One convenientmethod of charging is to expose the surface to be charged to a coronadischarge produced by connecting one or more fine wires to a directcircuit source, e.g., a direct circuit source of 3-10 kilovoltsnegative. The atmosphere surrounding the wires is ionized and the ionflow establishes the charge density on the surface. The charge decaysslowly in the dark over a period of minutes or hours depending upon thenature of the coating.

The recording element now sensitive to light, is exposed by any of theconventional photographic processes and the electrostatic charge isselectively leaked from the surface. The degree of charge decay on aparticular element is directly proportional to the amount of light towhich the area is exposed. If light is projected through a photographicnegative onto the charged surface, the largest amount of charge decaywill be in the area of the recording element corresponding to the mosttransparent areas of the film. Charge decay in other areas will becorrespondingly less depending upon the transparency of thecorresponding areas of the film. There is thus produced an electrostaticimage on the surface of the recording element which accuratelyreproduces the negative image on the film.

It is, of course, possible to effect selective charge decay by otherprocedures. For example, a contact print or drawing may be focused ontothe charged surface of the recording element with the aid of an opticalsystem using light from one or more incandescent lamps.

In one method of producing a lithographic plate, the insulating film inwhich the photoconductive material is suspended is a polymeric substancewhich is capable of further polymerization by the action of a catalystor by actinic light to produce a polymer of higher molecular weight.

In the next step of the process, the electrostatic image is converted toa polymeric image by controlled polym erization of the areas which haveretained a charge. This is often accomplished by depositing apolymerization catalyst such as finely divided aluminum octoate on theareas of the recording element bearing the electrostatic charge.Deposition may be effected, for example, by immersing the recordingelement in a developer bath comprising a liquid carrier in which thecatalyst is suspended. The liquid is characterized by a high electricresistivity and is usually an aliphatic hydrocarbon or a mixture ofaliphatic hydrocarbons including heptane and homologues thereof.Halogenated hydrocarbons may also be employed. The catalyst particles,by a process which is not completely understood, but is probably atriboelectric phenomenon, acquire a positive charge relative to thesurface of the recording element and are, therefore, attracted to theelectrostatic image and deposited on it.

The recording element with the catalyst selectively deposited on itssurface is removed from the developer bath and rinsed to remove anycatalyst particles which may have been deposited on the surface of therecording element in areas other than those bearing the electrostaticimage.

After rinsing, the recording element is'placed in a curing oven whereheat polymerization of the catalyzed areas takes place with the resultthat a polymeric image is produced corresponding to the originalelectrostatic image. The polymeric image comprises areas of highmolecular weight solvent resistant polymers surrounded by areas of lowmolecular weight solvent sensitive polymers. The background areas areselectively dissolved to expose the metal surface which may be etchedaccording to standard procedures after cleaning or descumming to producea photoengraving resist.

For the production of printed copper circuits the recording elementcomprises a photoconductive layer which is deposited on the coppersurface of a copper laminate and an electrostatic image corresponding tothe desired circuit formed thereon. The polymer image is formed and thebackground area dissolved as described above to leave the bare copper.This copper is then removed by etching, for example, with ferricchloride or ammonium persulfate solution. The polymer image which coversthe circuit is then removed with a solvent to leave bare copper in thedesired configuration for the electrical circuit.

In the electrostatic printing process the photoconductive substance,preferably finely divided zinc oxide in a polymeric film is coated onpaper. The electrostatic image is then produced by any convenientprocedure and a developer or toner, generally comprising a pigmented,low

melting resin powder on a carrier of iron particles, applied to thepaper in the charged areas with a magnetic brush. The resin particlesadhere to the charged areas by triboelectric deposition and the ironpowder remains on the magnetic brush. The paper is then heated to fusethe resin and produce the image.

All of these processes have the common element that a low molecularWeight polymer film containing a dispersed photoconductive material isformed on a base. The base may be either a metal such as zinc, magnesiumor copper, or it may be a fibrous substance especially cellulosic fibersas in a paper; The polymeric electric insulator is generally referred toas the binder and will be so identified hereinafter. The film isdeposited as a liquid photoconductive composition which contains thebinder in solution and the photoconductive substance in suspension. Itmay also contain other special purpose ingredients such as dyes toincrease the spectral range of the photoconductive material. Thecomposition is evenly distributed on the base, for example by sprayingor swirling, and the solvent evaporated to produce a recording element.

There are a number of problems which are common to these processes. Ithas been found that the binder film is susceptible to localizedbreakdown under theinfiuence of corona discharge. The result is thatpinholes are produced in the binder film and catalyst or toner are notdeposited, with the result that the ultimate image is marred.

Another problem is the problem of top voltage. The maximum voltage ofthe charge applied to the binder surface should be as reproducible aspossible to insure consistent printing density. With silicone binders,this so called top voltage is of the order of 400-600 volts. With otherbinders it it somewhat less. In any event as the recording element agesby storage before use, this top voltage changes.

Still another, is the apparently reduced charge capacity of therecording element under conditions of low relative humidity. The resultof this reduction is that the attainable print density may be reduced toonly 75% of the normal print density expected from the recording elementat 50% relative humidity.

It has been found that these problems are substantially alleviated bypreparing recording elements in accordance with this invention in whichan N-alkanol amine containing an aryl moiety is absorbed on the surfaceof the photoconductive substance. N-alkanol amines containing a phenylsubstituent are useful and of these tertiary-N- benzyl-N-alkanol aminesare particularly suitable. An especially preferred class of amines isrepresented by the formula (CH2) OH CuH CHgN wherein n is an integerfrom 1 to 3, Y is C H CH and alkyl group containing from 1 to 3 carbonatoms or (CH OH wherein n is an integer from 1 to 3. For reasons ofcommercial availability the preferred compounds within the scope of theabove formula are N,N- dibenzylethanol amine; N-methyl-N-benzylethanolamine and N,N-diethanol-benzyl amine.

The selected amine is preferably at least partially soluble in thesolvent for the binder so as to permit uniform absorption on thephotoconductor surface from solution. This is not essential, however,since the photoconductor can be independently activated. In one methodthe photoconductor is exposed to an atmosphere containing vapors of theselected amine. In another the photoconductor is suspended in a solutionof the amine and the solvent evaporated. The thus activatedphotoconductor is then resuspended in the binder solution.

If the wet development procedure is employed, i.e. if the developer isdeposited on the electrostatic image from a liquid medium, the amineshould, of course, be insolument technique utilizing a magnetic brush,for example is employed.

Any of the binder materials generally used in electrophotography areapplicable to the process of this invention although silicone resins arepreferred. Satisfactory binder materials include, forexample,polystyrene; acrylic and methacrylic ester polymers such as Acryloid A10and B72, polymerized ester derivatives of acrylic and methacrylic acids,both supplied by Rohm & Haas Company; Lucite 44, Lucite 45 and Lucite46, polymerized butyl methacrylates supplied by E. I. duPont de Nemours& Company; chlorinated rubber such as Parlon supplied by The HerculesPowder Company; vinyl polymers and copolymers such aspolyvinyl'chloride, polyvinyl acetate, etc. including Vinylite VYHH andVMCH manufactured by the Bakelite Corporation; cellulose esters andothers such as ethyl cellulose nitrocellulose, etc.; alkyd resins suchas Glyptal 2469 manufactured by The General Elec-v tric Company; Rezyl869, a linseed oil-glycerol alkyd provided by American Cyanamid Company;silicone resins such as DC 801, 804 and 996, all manufactured by the DowCorning Corp. or SR82 a methyl phenyl polysiloxane mixture availablefrom General Electric Company. This latter silicone resin is especiallyeffective in admixture with ethoxylated phenyl siloxanes, for example R830, available from Union Carbide Corp. Other binder mixtures containingone or more of the above mentioned binders, or other, may also beemployed.

The electrophotographic compositions of this invention comprisedissolved binder which may be partially polymerized as in the productionof a lithographic plate or substantially completely polymerized, as inthe electrofax process in a solvent containing dispersed photoconductorand from about 0.02 to about 0.08% of the selected amine which in thecomposition may be absorbed on the surface of the photoconductor.Suitable solvents include aliphatic or aromatic hydrocarbon solventscontaining, for example, up to eight carbon atoms. Benzene, toluene,methylene dichloride, ethylene dichloride, ethylene trichloride, carbontetrachloride, hexane, heptane and isooctane may be mentioned by way ofexample. The choice of solvents is not critical. Mixed solvents may beemployed.

The amount of zinc oxide or other photoconductor present in the liquidmay vary between rather wide limits. It may vary, for example, fromabout 7% to about 50% by weight based on the total weight of thecomposition. The total solids in the composition may vary from about 10%to about 60% and this will be substantially all binder andphotoconductor plus, in some cases minor amounts of other additives suchas dyes. The ratio of photoconductor to binder in the composition and inthe recording element is accordingly, from about 1.5/1 to 9/ 1.

A recording element of this invention comprises a base member, with asurface coating comprising aphotoconductor activated by absorbed aminedispersed in an electrically insulating binder. The percent by weight ofphotoconductor in the film is from about 50% to about by weight. Theratio of photoconductor to binder is from about 1.5/1 to about 9/1 andthe amine content is from about 0.02% to about 0.08% by weight based onthe weight of photoconductor.

The binder films on the base which are obtained using the compositionsof this invention are superior to previously known products in severalrespects. The. principal improvements are that the original charge maybe more uniformly dispersed on the surface coat and the changes in thecoated base due to aging are minimized.

The following examples are given by way of illustration only and are notto be construed as limitations of this invention, many apparentvariations of which are possible without departing from the spirit andscope thereof.

EXAMPLE 1 The photo-resist composition is prepared containing thefollowing ingredients and parts.

Ingredients: Parts SR-SZ 1 milliliters 630 Xylene do 900 Toluene do 100N,N-diethanolbenzyl amine do 0.3 Photoconductive zinc oxide (0.35 micronparticles size) grams 700 Uranine dye (1% in methanol) milliliters 18 1Asilicone binder available from the General Electric Company.

The amine is added to the binder and solvent composition before theaddition of the zinc oxide. The Zinc oxide is added to the mixturegradually and dispersed by high speed stirring for approximately twominutes. The dye is added drop-wise in 30 milliliters of toluene. Themixture is then stirred rapidly for approximately 3 minutes. It is usedto prepare a photoengraving or lithographic plate.

EXAMPLE II Part A The composition is prepared containing the followingingredients and parts.

Ingredients: Parts Pliolite S-7 grams 100 Toluene milliliters 150Activated hotoconductive zinc oxide grarns 100 I A styrene-butadienecopolymer available from Goodyear Tire 8; Rubber Company.

Part B The composition is a preparation of activated zinc oxide composedof the following ingredients and parts.

Ingredients: Parts Photoconductive zinc oxide grams 100 3 USP N0. 12available from New Jersey Zinc.

and is taken up in 200 milliliters of acetone containing 0.15 gram ofdissolved N,N-dibenzylethanol amine and the composition thoroughly mixedby high speed stirring. The acetone is evaporated and the zinc oxideheated for two hours at 250 F. until thoroughly dry.

I The mixture of Part A containing activated Zinc oxide from Part B isball-milled for six hours to produce an electrophotographic compositionsuitable for coating a base member to prepare a recording element.

EXAMPLE III The following components are mixed together in a high speedblender and applied to a roughened metal surface to provide therecording element.

Ingredients: Parts Methyl isobutyl ketone milliliters 500 Triphenylphosphate grams 1O N,N-diet-hanolbenzyl amine milliliters.. 0.05 SR-1844 do 18 Cellulose acetate butyrate (Eastman Grade Eab 500-1) grams 80Photoconductive zinc oxide do 250 4 A silicone binder available fromGeneral Electric Company.

EXAMPLE IV The following components are mixed in a high speed blender toprovide an electrophotographic composition which is useful for thepreparation of recording elements with an extremely smooth surfacecoating.

6 Ingredients Parts Freon TF milliliters 250 Lucite 46 grams 40N,N-dibenzylethanol amine do 0.10 Photoconductive zinc oxide do 100 5 Ahalogenated hydrocarbon available from E. I. du Pont. 6 A mctliacrylicester polymer available from E. I. du Pont.

EXAMPLE V An activated photoconductive zinc oxide is prepared bydispersing 100 grams of finely divided Zinc oxide in 190 milliliters ofacetone containing 0.05 milliliter of N,N- diethanolbenzyl amine andevaporation of the solvent. The thus treated oxide is taken up in 100milliliters of heptane containing 100 grams of Acryloid F-10 (availablefrom Rohm & Haas, Inc.) and the mixture thoroughly blended in a highspeed blender to provide an electrophotographic composition.

EXAMPLE VI An electrophotographic composition is prepared by thoroughlymixing the following components in a high speed blender.

Ingredients: Parts Photoconductive red mercuric oxide grams 100 R830 do40 Toluene milliliters 40 N,N-diethanolbenzyl amine gram 0.25

TA silicone binder available from Union Carbide.

EXAMPLE VII An electrophotographic composition is prepared by stirringthe following ingredients at high speed. Ingredients: PartsPhotoconductive zinc sulfide grams R830 8 do 30 N,N-dibenzyl amine do0.05 Toluene milliliters 5A silicone binder available from UnionCarbide.

EXAMPLE VIII An elcctrophotographic composition is prepared by mixingthe following ingredients in a high speed blender.

Lucite 46A methacrylic ester polymer available from E. I. du Pont.

The compositions of Examples VI, VII, and VIII are used to preparerecording elements using either zinc, magnesium or paper as the basemember.

What is claimed is:

1. An electrophotographic composition comprising a binder dissolved in avolatile solvent containing a photoconductor and an amine selected fromthe group consisting of N,N-dibenzylethanol amine,N-rnethyl-N-benzylethanol amine, and N,N-diethanolbenzyl amine, thetotal solids in the composition being from about 10% to about 60% byweight based on total weight, the ratio of photoconductor to binderbeing from about 1.5/1 to about 9/1, and the total amine in thecomposition being from about 0.02% to about 0.08% by weight based on theweight of photoconductor.

2. An electrophotographic composition comprising a binder dissolved in avolatile solvent containing photoconductive zinc oxide and an amineselected from the group consisting of N,I I-dibenzylethanol amine,N-methyl- N-benzylethanol amine and N,N-diethanolbenzyl amine, the totalsolids in the composition being from about 10% to about 60% by weightbased on the total weight, the ratio of zinc oxide to binder being fromabout 1.5/1 to 7 about 9/1, and the total amine in the composition beingfrom about 0.02% to about 0.08% based on the weight of zinc oxide.

3. An electrophotographic composition comprising a binder dissolved in avolatile solvent containing photo.- conductive zinc oxide together withN,N-dibenzylethanol amine, the total solids in the composition beingfrom about 10% to about 60% by weight based on the total weight, theratio of zinc oxide to amine being from about 1.5 1 to about 9/ 1, andthe total amine in the composition being from about 0.02% to about 0.08%based on the weight of zinc oxide.

4. An electrophotographic composition comprising a binder dissolved in avolatile solvent containing photoconductive zinc oxide together withN-methyl-N-benzylethanol amine, the total solids in the compositionbeing from about 10% to about 60% by weight based on the total weight,the ratio of zinc oxide to amine being from about 1.5/ 1 to about 9/ 1,and the total amine in the composition being from about 0.02% to about0.08% based on the Weight of zinc oxide.

5. An electrophotographic composition comprising a binder, dissolved ina volatile solvent containing photoconductive zinc oxide together withN,N-diethanolbenzyl amine, the total solids in the composition beingfrom about 10% to about 60% by weight based on the total weight, theratio of zinc oxide to amine being from about 1.5/1 to about 9/ 1, andthe total amine in the composition being from about 0.02% to about 0.08%based on the Weight of zinc oxide.

6. A recording element comprising a base member having a surface coatingcomprising an insulating binder having dispersed therein from about 50%to about 90% by weight of a photoconductor, said photoconductor beingactivated by absorption of from about 0.02% to about 0.08% by weight ofan amine selected from the group consisting of N,N-dibenzylethanolamine, N-methyl-N- benzylethanol amine and N,N-diethanolbenzyl aminebased on the weight of photoconductor, the ratio of photoconductor tobinder being from about 1.5/1 to about 9/ 1.

7. A recording element comprising a metallic base member having asurface coating comprising an insulating binder having dispersed thereinfrom about 50% to about 90% by weight of a photoconductor, saidphotoconductor being activated by absorption of from about 0.02% toabout 0.08% by weight of an amine selected from the group consisting ofN,N-dibenzylethanol amine, N-methyl- N-benzylethanol amine andN,N-diethanolbenzyl amine based on the weight of photoconductor, theratio of photoconductor to hinder being from about 1.5/ 1 to about 9/1.

8. A recording element comprising a paper base member having a surfacecoating comprising an insulating binder having dispersed therein fromabout 50% to about 90% by weight of a photoconductor, saidphotoconductor being activated by absorption of from about 0.02% toabout 0.08% by weight of an amine selected from the group consisting ofN,N-dibenzylethanol amine, N-methyl- N-benzylethanol amine andN,N-diethanolbenzyl amine based on the weight of photoconductor, theratio of photoconductor to binder being from about 1.5/ 1 to about 9/ 1.

9. A recording element comprising a base member having a surface coatingcomprising an insulating binder having dispersed therein from about 50%to about 90% by weight of photoconductive zinc oxide activated byabsorption of from about 0.02% to about 0.08% by weight of an amineselected from the group consisting of N,N-

zinc oxide, the ratio of zinc oxide to binder being from about 1.5/1 toabout 9/1.

11. A recording element comprising a base member having a surfacecoating comprising an insulating binder having dispersed therein fromabout 50% to about by weight of photoconductive zinc oxide activated byabsorption of from about 0.02% to about 0.08% by weight ofN-methyl-N-benzylethanol amine based on the weight of zinc oxide, theratio of zinc oxide to binder being from about 1.5/1 to about 9/1.

12. A recording element comprising a base member having a surfacecoating comprising an insulating binder having dispersed therein fromabout 50% to about 90% by weight of photoconductive zinc oxide activatedby absorption of from about 0.02% to about 0.08% by weight 1 ofN,N-diethanolbenzyl amine based on the weight of Zinc oxide, the ratioof zinc oxide to binder being from about 1.5/1 to about 9/1.

13. A photoconductor for use in the preparation of recording elementsfor electrophotography comprising a normally photoconductive materialactivated by absorption of from about 0.02% to about 0.08% by weight ofan amine selected from the group consisting of N,N-dibenzylethanolamine, N-methyl-N-benzylethanol amine and N,N-diethanolbenzyl amine.

14. A photoconductor for use in the preparation of recording elementsfor electrophotography comprising a photoconductive zinc oxide activatedby absorption of from about 0.02% to about 0.08% by weight of an amineselected from the group consisting of N,N-dibenzylethanol amine,N-methyl-N benzylethanol amine and N,N-diethanolbenzyl amine.

15. A photoconductor for use in the preparation of recording elementsfor electrophotography comprising a photoconductive zinc oxide activatedby absorption of from about 0.02% to about 0.08% by weight of N,N-di'benzylethanol amine.

16. A photoconductor for use in the preparation of recording elementsfor electrophotography comprising a photoconductive zinc oxide activatedby absorption of from about 0.02% to about 0.08% by weightof N-methyl-N-benzylethanol amine.

17. A photoconductor for use in the preparation of recording elementsfor electrophotography comprising a photoconductive zinc oxide activatedby absorption of from about 0.02% to about 0.08% by weight ofN,N-diethanolbenzyl amine.

References Cited UNITED STATES PATENTS 3,197,307 7/1965 Blakeet a1.96-1.8 3,250,613 5/1966 Gramza et al 96l.8

NORMAN G. TORCHIN, Prihzary Examiner.

C. E. VAN HORN, Assistant Examiner,

1. AN ELECTROPHOTOGRAPHIC COMPOSITION COMPRISING A BINDER DISSOLVED IN AVOLATILE SOLVENT CONTAINING A PHOTOCONDUCTOR AND AN AMINE SELECTED FROMTHE GROUP CONSISTING OF N,N-DIBENZYLETHANOL AMINE,N-METHYL-N-BENZYLETHANOL AMINE, AND N,N-DIETHANOLBENZYL AMINE, THE TOTALSOLIDS IN THE COMPOSITION BEING FROM ABOUT 10% TO ABOUT 60% BY WEIGHTBASED ON TOTAL WEIGHT, THE RATIO OF PHOTOCONDUCTOR TO BINDER BEING FROMABOUT 1.5/1 TO ABOUT 9/1, AND THE TOTAL AMINE IN THE COMPOSITION BEINGFROM ABOUT 0.02% TO ABOUT 0.08% BY WEIGHT BASED ON THE WEIGHT OFPHOTOCONDUCTOR.